SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1200 AM CST Sun Nov 29 2020 Valid 291200Z - 301200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE CENTRAL AND EASTERN GULF COAST REGION...AND THE SOUTHEASTERN U.S. ATLANTIC COAST... ...SUMMARY... Isolated strong to severe thunderstorms remain possible today and tonight across southern portions of the Gulf Coast states into the eastern Carolinas. A couple of tornadoes, and a few locally damaging wind gusts, are the main threats. ...Synopsis... An upper low initially over Oklahoma is forecast to move eastward to the mid/lower Mississippi Valley through the first half of the period, but will gradually become absorbed by a northern-stream trough digging east-southeastward out of the northern Plains/upper Mississippi Valley into the Midwest/middle Mississippi Valley overnight. At the surface, a weak low is forecast to move across the northern Gulf of Mexico near the Louisiana coast toward Mobile Bay during the day, along a pre-existing baroclinic zone. Thereafter, the low is progged to turn northeastward and move quickly northward along the Appalachians, deepening rapidly as it reaches the West Virginia vicinity by the end of the period/Monday morning. ...Central and eastern Gulf Coast/southeastern Atlantic coast... Widespread precipitation is forecast to be ongoing at the start of the period ahead of the advancing upper low, from southern and eastern Texas across Arkansas and Louisiana into southern Mississippi and vicinity. The substantial precipitation and cloud cover -- that should persist through the day -- over the central Gulf Coast states/mid South should maintain a stable boundary layer. Any potential for surface-based convection appears likely to remain confined to southeast Louisiana, southeast Mississippi, and into southern Alabama and the Florida Panhandle through the daylight hours. A separate area of storms may evolve during the afternoon across eastern South Carolina and into the southern Atlantic, near a developing warm front. During the evening and overnight, as the surface low deepens/shifts rapidly north-northeastward, surface-based convection will spread eastward across southeastern Alabama and roughly the southern half of Georgia, and northern portions of the Florida Peninsula ahead of the strengthening cold front. Meanwhile, surface-based convection will shift northward across the eastern Carolinas and eventually into southeastern Virginia, east of the surface low as the warm front shifts northward. Instability is forecast to remain quite limited inland, with generally less than 300 to 500 J/kg mixed-layer CAPE expected. However, increasingly strong and veering flow with height will evolve with time, in tandem with surface cyclogenesis, yielding shear profiles sufficient for supercells. While modest CAPE should remain a limiting factor, a couple of tornadoes will be possible, within a broad SLGT risk area from southern Mississippi east to northern Florida, and north across the eastern Carolinas to southeast Virginia. A few stronger storms may also produce gusty/locally damaging winds, particularly if small bowing segments can evolve locally. Risk will taper off from west to east along the Gulf Coast through the evening, but may linger through the end of the period across portions of Florida, and particularly along the Atlantic Coastal portion of the outlook. ..Goss/Moore.. 11/29/2020
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